1. Field of the Invention
This invention relates to conductive elements e.g. heating elements, comprising a conductive composition deposited onto a support.
2. Discussion of the Prior Art
Conductive elements comprising laminates of a porous electrically conductive layer composed of electrically conducting particles bonded together in an open continuous structure, electrodes on the conductive layer in electrical contact therewith and at least one layer covering each surface of the conductive layer are disclosed in U.S. Pat. No. 2,952,761 to R. Smith-Johannsen. Numerous methods for the preparation of similar elements for use as heaters are disclosed in the prior art. See, for example, U.S. Pat. Nos. 2,803,566, 2,891,228 and 2,991,257 all to R. Smith-Johannsen, 3,400,254 to Takemori, 3,876,968 to R. D. Barnes et al. 3,900,654 to J. J. Stinger and 4,169,911 to Yoshida et al. The disclosures of the above patents are incorporated by reference herein.
In the prior art processes the conductive composition is applied to a substrate, for example, asbestos fiber mat, glass fabric or thermoplastic film. The structure is then generally laminated to one or more outer layers. When the conductive composition is an aqueous based dispersion of conductive particles, preferred for reasons of economy and safety, as in above-mentioned U.S. Pat. No. 2,952,761, the substrate used in actual practice has generally been an asbestos fiber mat, although use of other substrates is suggested in this patent. When the conductive composition is one containing an organic solvent medium, a wider range of substrates, including thermoplastic films, can be used. For example, in U.S. Pat. No. 3,900,654, a heating element is prepared from a thermoplastic film support with an adherent conductive layer of an electrically conductive material containing carbon black dispersed in a fluorocarbon elastomer. It is mentioned that the support can be composed of a layer of the polymer adhered to another material such as a fibrous sheet. It is also mentioned that the conductive elastomer can be applied from a liquid coating composition in which the elastomer is dispersed in an organic solvent or in water.
Conductive elements prepared by processes of the type described above are not entirely satisfactory. While a very useful product can be made by impregnating a uniform asbestos fiber mat with an aqueous dispersion of conductive particles, careful precautions are necessary in handling products including asbestos fibers, and attempts to replace the uniform asbestos fiber mat by other uniform fiber mats have not yielded satisfactory products. More particularly, it is often difficult to obtain good adhesion of the conductive particles to the substrate coated, particularly when the substrate is flexible and/or is not adequately wetted by the liquid composition comprising conductive particles. If wetting is inadequate, so-called "mud-cracking" of the conductive layer can take place when the liquid composition is dried, resulting in unstable electrical properties. Even if satisfactory adhesion can be obtained initially, it is difficult to make a product which has stable electrical properties over an extended period of use, especially when the element is subject to flexing.